The present invention relates to reciprocating electromagnetic actuators. More particularly, the present invention relates to a reciprocating electromagnetic actuator that produces instantaneous output forces over the full stroke or arc of the reciprocating device that are linearly proportional to an applied input current.
Electromagnetic actuator devices are known in the art that produce reciprocating motion. Such reciprocating motion may be either linear, i.e., back and forth along a straight-line axis; or angular, i.e., back and forth along a curved or arched axis. (It is noted that the term "linear" as used herein may have two separate meanings. When used to describe motion, "linear" refers to motion along a straight-line axis. When used to describe forces, "linear" refers to a proportional relationship between the output force of the device and an applied input current. More particularly, if F=kI, where F is the output force, I is the input current, and k is a constant, the output force is said to be "linear".) However, most are characterized by very small stroke, low frequency response, or low efficiency (low output power relative to the input power and weight/size of the device).
One of the most commonly known devices for producing linear motion is the voice coil motor. The voice coil motor typically includes an electric coil in the form of a thin wall cylinder that fits into a co-axial annular air gap in a magnetic circuit. The magnetic circuit includes a magnet (usually a permanent magnet) for generating magnetic flux that passes across the annular air gap. The voice coil is guided to move axially at right angles to the magnetic flux in the annular air gap.
While the voice coil motor offers the advantage of a relatively high frequency response, it suffers from numerous drawbacks. For example, because the voice coil is trapped in still air between the sides of the air gap, the coil exhibits poor heat dissipation. Further, the air gap thickness must equal or exceed the thickness of the coil plus mechanical clearances on each side. Large air gaps require large magnets in order to maintain the same forces that could be generated using small air gaps and smaller magnets. Typically, the coil is made thin to minimize magnet size at the expense of making the coil resistance high and making electrical heating correspondingly high.
Also known in the art for producing angular reciprocating motion is the d'Arsonval galvanometer. This device forms the basis for most DC voltmeters and ammeters. It is essentially the rotary equivalent of the voice coil motor. As such, it has the same advantages and disadvantages.
Still another type of device known in the art for producing linear motion is that shown in U.S. Pat. No. 3,336,488, invented by Scott, and that shown in U.S. Pat. No. 3,366,809, also invented by Scott. The Scott devices teach the use of a magnetic circuit having a stator with at least two pole pieces and an armature adapted for movement relative to the stator. Each pole piece has at least one slot therein, thereby forming at least two teeth in each pole piece through which the magnetic flux can flow. In particular, Scott teaches the concept of carefully spacing the teeth in the pole piece relative to the length of the armature segments facing the pole piece so that flux in the magnetic circuit is alternately transferred from one tooth to the next as the armature moves. The advantages of the Scott devices are that a long stroke can theoretically be achieved by simply increasing the number of teeth. However, the disadvantages of the Scott devices are that: (1) the flux density across the air gap does not remain constant as the armature moves; (2) the forces developed are thus non-linear (not proportional to input current); and (3) this non-linearity has the effect of adding a centering force to the intended force, as described below.
To illustrate, in the Scott devices the applied current superimposes a local magnetic flux on the main magnetic flux (from the permanent magnet). When the moving core (armature) is centered, the local flux is a maximum; but as soon as the core displaces, the local flux decreases. This is because the total reluctance of the local flux circuit is the sum of the reluctance on each side of the slot. The reluctance on the side with diminishing overlap approaches infinity as the moving core edge approaches the slot edge. Thus, the total reluctance of the local flux circuit also approaches infinity as the moving core edge approaches the slot edge. Thus, the total reluctance of the local flux circuit also approaches infinity, causing the flux density across the gap to decrease to zero as the core edge approaches the slot edge. This action, in turn, creates a non-linear output force which has the effect of centering the moving core between the two teeth at each end of the slot. Further, if large currents are applied to the Scott devices in an attempt to generate large forces, the iron will saturate and demagnetize the magnets.
In general, therefore, the Scott devices are useful only for applications where a non-linear output force is acceptable for generating reciprocating motion at relatively low output forces, such as in electric cutting devices. The Scott devices are totally inadequate for applications requiring a linear output force independent of the position of the moving core (armature), particularly where such forces must be large forces.
Another type of linear motion reciprocator known in the art is taught in U.S. Pat. No. 4,349,757, invented by Bhate. The Bhate device incorporates a series of carefully spaced permanent magnets on the armature, having alternating radially oriented polarities. The magnets are adjacent to the air gap. While the Bhate device offers some advantages, a careful examination thereof shows that the flux density at each point in each magnet rises and falls as that point is adjacent to a tooth or slot of the pole piece. That portion of the magnet opposite the slot is useless. Further, the rise and fall of the flux density tends to demagnetize the magnet. What is needed therefore, is a permanent magnet reciprocating device wherein the magnetic flux density remains constant, thereby providing linear forces and avoiding undesirable demagnetization.